cobol: Honor explicit -ffree-form

Previously, an explicit format was only determined as long as
command_line_indicator_column did not equal 0. However, OPT_ffree_form
assigns it to 0 via cobol_set_indicator_column, and therefore the
explicit user setting was being ignored, falling back to the default
behaviour, a heuristic where the first {PROGRAM,FUNCTION}-ID is
examined to determine whether either fixed or free format should be
applied.

Since comparing against 0 is not enough to determine whether a given
format was explicitly requested, a boolean flag has been added to
force a call to source_format_t::indicator_column_set whenever
-ffree-form is requested.

gcc/cobol/ChangeLog:

	* lexio.cc (cdftext::lex_open): Add explicit_format condition.
	(cobol_set_indicator_column): Use cdftext::explicit_format.
	* lexio.h (class cdftext): Add cdftext::explicit_format.